The cyber attack is believed to have been developed by using a leaked code from the United States National Security Agency.

Mr Martin, who leads the National Cyber Security Centre - part of GCHQ - said "thousands of organisations and individuals in dozens of countries" had been hit by the attack.

Britain's opposition Labour Party said the attack on English hospitals showed the need to place cyber security at the heart of government policy.

Doctors reported seeing computers go down "one by one" as the "ransomware" took hold on Friday, locking machines and demanding money to release the data. While cyber security firm Kaspersky Lab said that at least 45,000 computers in 74 countries were affected by the "ransomware", another security firm Avast put the tally at 57,000 systems across 99 countries.

The unprecedented attack plunged the health service into chaos, forcing hospitals to cancel and delay treatment.

The US Department of Homeland Security said late on Friday that it was aware of reports of the ransomware, was sharing information with domestic and foreign partners and was ready to lend technical support. Portugal Telecom and Telefonica Argentina both said they were also targeted.

Still, only a small number of US-headquartered organisations were hit because the hackers appear to have begun the campaign by targeting organisations in Europe, said Vikram Thakur, research manager with security software maker Symantec.

Microsoft said it was pushing out automatic Windows updates to defend clients from WannaCry.

Authorities in Britain have been braced for cyber attacks in the run-up to the vote, as happened during last year's USA election and on the eve of the French vote.

The demand is for a payment of $300 (£230) in virtual currency Bitcoin to unlock the files.

Hospitals across England reported the cyber attack was causing huge problems to their services and the public in areas affected were being advised to only seek medical care for emergencies. The interior ministry said on its website that about 1,000 computers had been infected but it had localized the virus.

She told BBC Radio 4's Today programme the virus had not been targeted at the NHS, saying the attack "feels random in terms of where it's gone to and where it's been opened".

'Seeing a large telco like Telefonica get hit is going to get everybody anxious.

"It certainly caused a meltdown in their [hospitals'] email servers, crashed computers and then a message came up with a threat of a ransom, that all the files will be deleted permanently if the payment was not made", Al Jazeera's Sonia Gallego, reporting from the United Kingdom capital, London, said.